In wireless communication systems, a base station transmits information to a subscriber station in a “downlink” and receives information from the subscriber station in an “uplink.” As used herein, the term subscriber station includes both fixed and mobile stations, including, but not limited to, wireless data devices, wireless voice devices and/or the like. There are a variety of different modulation and transmission schemes for wireless communications. One type of wireless communications is defined in the IEEE 802.16e standard, which uses an orthogonal frequency division multiple access (OFDMA) frame with Time Division Duplexing (TDD) as a modulation and transmission scheme.
FIG. 1 illustrates an exemplary OFDMA frame with TDD used in the IEEE 802.16e standard. The horizontal axis represents time, while the vertical axis represents frequency tones or subcarriers. An OFDMA frame in a TDD system is divided into downlink and uplink subframes. Between the downlink and uplink subframes of a particular frame is a transmit transmission gap (TTG), which provides time for a base station to transition from a transmission mode to a receive mode and subscriber stations to transition from a receive mode to a transmission mode. A receive transmission gap (RTG) is provided between an uplink subframe of one frame and the downlink subframe of a subsequent frame. The RTG provides base stations time to transition from a receive mode to a transmission mode and subscriber stations to transition from a transmission mode to a receive mode.
In the frame of FIG. 1, the downlink subframe includes a preamble, frame control header (FCH), downlink MAP (DL-MAP), uplink MAP (UL-MAP) and downlink data channels. The preamble is used by subscriber stations for cell acquisition and frame synchronization. The FCH is a broadcast channel located right after preamble, and includes information indicating the size of DL-MAP, repetition coding and forward error correction (FEC) coding used in DL-MAP, and other information related to the current frame. The DL-MAP, which follows immediately after FCH, includes a number of information elements (IEs), which define the usage of the downlink data channels and includes information such as frame synchronization, paging messages, downlink channel allocations and configuration change counts.
The UL-MAP includes a number of information elements, which define the usage of the uplink data channels, and includes information such as uplink channel allocations, and uplink configuration change counts. The downlink data channels are used to transport information from a base station to subscriber stations. The uplink subframe includes the uplink data channels, as well as ranging and uplink control channels. The uplink data channels are used to transport information from subscriber stations to a base station.